A. Origin of the Invention
The invention described herein was made by an employee of the United States Government and may be manufactured and used by and for the Government for Government purposes without payment of any royalties thereon or therefore.
B. Field of Invention
This invention pertains to the art of modified silicate clays for dispersion in polymeric matrices. In particular, the present invention is directed to a modified silicate clay formed by a co-ion exchange process with a mono-protonated diamine and alkyl ammonium ions.
C. Description of the Related Art
The nanometer level dispersion of a layered silicate into a polymer matrix typically requires organic modification of the silicate interlayer surface. This modification is easily achieved by ion exchange of the interlayer metal cations found naturally in the silicate, with a protonated organic cation, typically a protonated amine. Long chain alkyl ammonium ions are commonly chosen as the ion exchange material as they effectively lower the surface energy of the silicate and ease the incorporation of organic monomers or polymers into the silicate galleries.
However, in high temperature thermosetting polymers, use of long chain alkyl ammonium ions poses two problems. First, the decomposition temperature of the alkyl chain is commonly lower than the processing temperature of the polymers. Degradation of the organic modifier can result in aggregation of the silicate layers leading to micron level dispersion, rather than nanometer level dispersion of the individual clay platelets. Secondly, the presence of the flexible chains in the rigid polymer matrix has plasticizing effects which can lower the strength and use temperature of the material.
Previous work has demonstrated that ion exchange with an aromatic diamine is a viable method of achieving irreversible swelling of the silicate in a thermoplastic polyimide matrix. However, in a thermosetting polyimide, silicate modification with only the diamine tends to increase the melt viscosity of the system and may affect the polymer structure on curing. As a result, dispersion of this organically modified silicate into a thermosetting polymer matrix may not enhance polymer properties such as strength, modulus, or thermal stability.
The present invention provides a synergistic co-ion exchange process using a mono-protonated aromatic diamine and alkyl ammonium ions. A relatively low level of alkyl ammonium ions is utilized to minimize oligomer melt viscosity without the downfalls of low degradation temperature and plasticization effects.
The co-ion exchange of a protonated aromatic diamine and an alkyl ammonium ion into a silicate clay is unique. This organic modification optimizes the benefits of silicate dispersion into a highly-crosslinked, thermosetting polymer. Co-ion exchange allows irreversible swelling of the silicate layers under elevated processing temperatures, without adversely increasing the melt viscosity. Therefore, the present invention provides a nanocomposite exhibiting improved properties and unexpected results.
According to one aspect of the present invention, a new and improved clay/organic chemical composition is provided. The clay/organic chemical composition for dispersion in a polymer matrix, wherein the clay/organic chemical composition comprises an ion-exchanged reaction product produced by the intercalation and reaction of:
(a) a clay having a cation exchange capacity;
(b) a synergistically effective amount of alkyl ammonium ion to ion-exchange with interlayer cations to provide minimized oligomer melt viscosity of the polymer matrix without adverse plasticizing effects; and,
(c) a synergistically effective amount of a mono-protonated aromatic diamine to provide irreversible swelling of the polymeric matrix, by providing a free amine site that can react with polymer precursors.
According to another aspect of the invention, there is provided a nanocomposite comprising:
(a) a polymer matrix comprising the polymerization of monomer reactants where the monomers are 3,3xe2x80x2,4,4xe2x80x2-benzophenonetetracarboxylic acid, methylene dianiline, and 2-carbomethoxy-3-carboxy-5-norbornene (hereinafter, PMR-15); and
(b) a layered clay material dispersed in the polymer matrix, the layered clay material being a clay/organic chemical composition comprising an ion-exchanged reaction product produced by the intercalation and reaction of:
(i) a clay having a cation exchange capacity;
(ii) a synergistically effective amount of alkyl ammonium compound to ion-exchange with interlayer cations to provide minimized oligomer melt viscosity of the polymer matrix without adverse plasticizing effects; and,
(iii) a synergistically effective amount of a mono-protonated aromatic diamine to provide irreversible swelling of the polymeric matrix.
According to another aspect of the invention there is provided a process for producing a nanocomposite comprising the steps of:
(a) modifying a layered clay material by co-ion exchange with an alkyl ammonium ion and a mono-protonated aromatic diamine;
(b) dispersing the modified layered clay material into a mixture of monomers; and
(c) in-situ polymerizing the monomer to obtain a composite having the modified clay material dispersed in a polymeric matrix.
According to another aspect of the invention, there is provided a clay/organic chemical composition formed by reaction of a mono-protonated aromatic di amine and an alkyl ammonium ion with a silicate clay having exchangeable metal cations within the silicate galleries at a plurality of cation exchange sites where interlayer cation exchange takes place, the clay/organic chemical compound comprising:
(a) surface modification by ionic interaction at the silicate surface;
(b) a free (react-able) amine site;
(c) a mono-protonated diamine ion exchanged at a first cation exchange site; and,
(d) an alkyl ammonium ion exchanged at a second cation exchange site.
According to another aspect of the invention, there is provided a process for synthesizing a silicate clay reinforced polymer comprising the steps of:
(a) providing a silicate clay having an overall negative charge and interlayer cation exchangeable sites;
(b) modifying the silicate surface by interaction of one amine group of a mono-protonated aromatic diamine with the silicate surface and leaving one amine group free for subsequent interaction;
(c) exchanging cations at the cation exchangeable sites with alkyl ammonium ions;
(d) providing a monomer capable of interaction with the free amine group; and,
(e) in situ polymerizing a monomer.
Definitions:
Whenever used in this specification, the terms set forth shall have the following meanings:
xe2x80x9cTgxe2x80x9d refers to a xe2x80x9cglass transition temperaturexe2x80x9d. A preferred method of measuring the Tg is by using a differential scanning calorimeter at a heating rate of 10xc2x0 C. per minute. xe2x80x9cLayered clay,xe2x80x9d xe2x80x9clayered clay material,xe2x80x9d or xe2x80x9cclay materialxe2x80x9d refers to any organic or inorganic material or mixtures thereof, such as smectite clay mineral, which is in the form of a plurality of adjacent, bound layers. The layered clay comprises platelet particles and is typically swellable.
xe2x80x9cPlatelet particles,xe2x80x9d xe2x80x9cplateletsxe2x80x9d or xe2x80x9cparticlesxe2x80x9d shall mean individual or aggregate unbound layers of the layered clay material.
xe2x80x9cDispersionxe2x80x9d or xe2x80x9cdispersedxe2x80x9d is a general term that refers to a variety of levels or degrees of separation of the platelet particles. The higher levels of dispersion include, but are not limited to, xe2x80x9cintercalatedxe2x80x9d and xe2x80x9cexfoliated.xe2x80x9d
xe2x80x9cIntercalatexe2x80x9d is defined as the situation where a material is inserted between the platelets or layers of another material. When a material is intercalated between the platelets of clay it means the material is inserted between the layers such that the distance between the platelets of clay is increased a measurable amount.
xe2x80x9cExfoliatexe2x80x9d or xe2x80x9cexfoliatedxe2x80x9d shall mean platelets dispersed predominantly in an individual state throughout a carrier material, such as a matrix polymer.
xe2x80x9cNanocompositexe2x80x9d or xe2x80x9cnanocomposite compositionxe2x80x9d shall mean a polymer or copolymer having dispersed therein a plurality of individual platelets obtained from a layered clay material.
xe2x80x9cMatrix polymerxe2x80x9d shall mean a polymer in which the platelet particles are dispersed to form a nanocomposite.
xe2x80x9cCo-ion exchangexe2x80x9d shall mean a process for exchanging the interlayer cations of a layered silicate with more than one type of organic material, either simultaneously or sequentially.
The invention is directed to co-ion exchange of the interlayer cations of a layered silicate with a mono-protonated aromatic diamine and an alkyl ammonium ion. The mono-protonated aromatic diamine essentially tethers one end of the diamine to the silicate, leaving the second amine free for reaction with monomers during polymer synthesis. The presence of the diamine allows chemical reaction between the silicate surface modification and the monomers. This reaction strengthens the polymer silicate interface, and ensures irreversible separation of the individual silicate layers.
Incorporation of the alkyl ammonium ions into the silicate galleries helps to keep the oligomer melt viscosity low during processing.
The presence of the mono-protonated aromatic diamine and the alkyl ammonium ion provide unexpected synergistic results. For example, evaluation of polymer thermal oxidative stability and mechanical properties demonstrates that modification of the silicate surface by co-ion exchange provides superior polymer properties over nanocomposites prepared with traditionally modified silicates.
Useful clay materials include natural, synthetic, and modified phyllosilicates. Illustrative of such natural clays are smectite clays, such as montmorillonite, saponite, hectorite, mica, vermiculite, benonite, nontronite, beidellite, volkonskoite, magadite, kenyaite, and the like. Illustrative of such synthetic clays are synthetic mica, synthetic saponite, synthetic hectorite, and the like. Illustrative of such modified clays are fluorinated montmorillonite, fluoronated mica, and the like.
In the preferred embodiment, the inventive polymer matrix may comprise PMR-15, addition-type thermosetting polyimides, thermoplastic polyimides, and mixtures thereof.
This invention is illustrated by the following examples that are merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it can be practiced.